There is no best - period. You should look for the largest caliber/heaviest bullet that YOU can fire quickly and accurately. That may change over time as you get more used to shooting and can handle more - or less as you get older and parts start to wear out. With the new crop of bullets out there the 9mm is no longer too small. Many people (I'm one of them) don't like the .40 S&W because the recoil is too sharp, and on me the pain is debilitating in my wrist area, I shoot a .45 ACP or .38 Super and I do A LOT more practicing with a .22 conversion kit on either gun than I ever did in the past. Do what works for you, not what someone else says is the best for them.

The 9mm has been around a long time. It did not have a good reputation as a SD round, real or imagined, mainly due to the availability of decent SD ammo.

With today's modern offerings it is on par with anything else.

The .40 caliber came about after the FBI had developed and started using the 10mm but found the guns to big and the recoil to sharp for many of their agents. S&W then came up with shortening the 10mm case and it would fit into a 9mm size gun. It offered a bigger bullet and was on par with 9mm magazine capacity and it took off from there.

It has been touted as the perfect self defense cartridge simply because it is bigger than a 9mm and smaller than a .45 so it should be right for everyone. There have always been two basic camps in regards to SD pistol calibers. Those that prefer small lightweight, fast moving bullets (9mm) and those that prefer bigger and heavier, slow moving bullets (.45). Over the years 9mm bullets have gotten bigger and slower like the 147 grain and .45 bullets have gotten lighter and faster like the 165 grain.

The .40 caliber fills the void, if there really is one, between the two. I think, someone please chime in here, the lightest .40 caliber bullet is 135 gr and the heaviest is 180 to 200 grain range. I don't follow the .40 caliber so not sure on that. Generally both the 9mm and .40 operate in the same pressure ranges. Recoil is "sharper" than a 9mm as has been stated but not uncontrollable by any means.

There are many criteria that people use to determine what is the best SD caliber. Bullet weight, recoil, magazine capacity, ballistics and so on. To me where you place the bullet is king no matter what the caliber is. Yes the bullet must be effective enough to penetrate to a depth to reach vital organs but the size of the bullet, again to me, does not make that much difference as long as it is at least a service caliber bullet.

Manufactures of SD ammo like any other company selling something always come up with new designs stating that their bullet is better than X because it is shiny, has grooves in it and has a deadly new name. In reality enough of any good modern soft point or hollow point put in the right spot will accomplish the job.

9mm ammo can be had for cheaper practice but also keep in mind that some .40 caliber pistols can be converted to 9mm for cheaper practice.

- Prior to developing the .40 cartridge, the FBI had requested the full-power 10mm ammunition be reduced slightly to make it more controllable, giving rise the the so-called "FBI Lite" 10mm load (180-200 grain bullet at about 950-1000 FPS). It was this reduced-power 10mm load that S&W decided they could squeeze into a shortened case, making it possible to use it in 9mm-sized guns. The .40 S&W will always be significantly less powerful than any full-throttle 10mm ammo (giving rise to any early nickname that 10mm shooters applied to the .40 cartridge -- the .40 Short & Weak).

- The .40 S&W was primarily designed as a police cartridge, to address several police-centric problems. One, as said above, was to fill the gap between the "lottsa little high-speed bullets" 9mm fans, and the "manstopping hunka-chunka lead" big-bullet .45 fans. Several other concerns were that .45 fans weren't particularly fond of the hi-capacity DA/SA wondernine guns; most police administrators didn't want cops walking around with cocked-and-locked 1911 autopistols on their belts (which was the most common .45 bullet delivery system available at the time); and all the .45 guns (and some of the 9mm DA/SA guns) had large frames/grips that were hard for small-statured officers to manage. The new crop of .40 caliber Safe-Action Glocks, or DAO modifications of various S&W autopistols, gave administrators an "out" that didn't require one side to win-out over the other. Everyone got new (supposedly safer/easier-to-use) guns that fit the hands of the smaller officers much better; the .45 fans got a bullet that still started with a ".4", and weighed nearly as much as the lightweight .45 JHPs; and the 9mm fans still had nearly the same magazine capacity (10 to 15 rounds of .40, vs. 14 to 17 shots in 9mm) that they liked and wanted to keep.

To answer the original poster's question, I'm generally a fan of the 9mm first and foremost, but I have several .40s as well. Lower recoil and lower ammo costs are both important for new shooter development, and I personally don't think the .40 offers any real advantage when you include the increased costs (both the monetary cost of "fewer bangs for the buck", and less accurate shooting due to this reduced amount of practice/training), along with slower followup shots (also made worse by the reduced practice/training time/shots investment).

I shoot the .40 well, but I shoot the 9mm better, and faster. Confidence is hard to put a set value on, but more is better.

"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)

Oh boy, the caliber wars. The venerated 9mm, .40S&W, and .45ACP are and remain the most widely used semi-auto calibers out there. So which is best. Strictly speaking, one should chose the most powerful caliber and load with which one can reliably, consistently, and confidently deliver rounds to target, time after time. Now far as capability of these calibers, keep in mind that the factors which are paramount in stopping a deadly attack are singly or in combination; blood loss, central nervous system disruption, or major bone structure damage. In order to do any of these things, a bullet must be able to penetrate deep enough into a body to effect the necessary damage. The next time you are out in public, find a rather large young man and imagine him taking a round to his chest. That bullet has to penetrate clothing, possibly bone, muscle, and then still have enough energy to get well within the chest cavity to do damage to blood vessels, vital organs, and perhaps the spinal column. A difficult job for a pistol cartridge.

Next, do some research and read and watch what medical professionals have to say about this topic. I have done this and the ones I found all indicated that the larger and heavier bullets tended to fair quite a bit better than the smaller and lighter ones. I viewed X-rays, wound photos, and read many pages of reports and comments about this subject. So what do I carry and train with?

I prefer the .40S&W in a premium law enforcement loading out of one of my gen3 Glock 23's. However, I also carry from time to time a premium law enforcement 9mm load when I deem it to be in my interests at the time. I train with both and an proficient with both. The bottom line is what I wrote above. Pick the caliber and load with which you can consistently and confidently deliver rounds to target. If that happens to be a 9mm, then so be it. If it is a .40 or a .45, then so much the better. In any of these calibers, make sure your SD ammunition is top drawer and proven on the street.

I have both - both are fine if you can put rounds on target, when attacked.

I prefer .45 ACP, because I just like 'em. But ask anybody who is experienced with gunshot wounds and they will tell you that they are all devastating to major organs, the pulmonary system, and the central nervous system, and that all are probably survivable, if fired into most other locations.

I have both and can shoot well with them. I think it all comes down to shot placement. If you can't hit the bad guy you may as well be throwing stones at him or her. Get what ever you can shoot the best and feels good to you in your hand.

I prefer a .357 Magnum revolver as my defense gun. I keep my Ruger GP100 beside the bed and I use my Ruger SP101 as my CCW gun. There is nothing like a.357 magnum loaded with Hornady Custom .357 Mag 158gr HP/XTP for self defense.